Huawei HG530 Installation Instructions Manual page 12

ISDN Line
Disconnect the cable that connects the telephone
set to the ISDN equipment (N.T. 1) from the telephone
socket and connect it to the black port on the splitter.
A. Using the blue cable, connect the blue port on the
splitter to the telephone socket.
B. Next, use the grey cable to connect the grey port
on the splitter to the grey port on the Conn-x equipment.
Note: If your telephone network is connected to any device
(alarm, telephone exchange, etc.), you must use an additional
ADSL lter or splitter. Please contact a specialized installer.